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The problem I see with doing this is that if take for example a thread which was originally created 10 years ago is bumped a few days ago would appear to be a new thread, even though the content is likely to be 10 years old.
that requirement is the reason why i asked for this feature. i have few threads that are running for more than a few years (based on latest updates of software and web services) and they do not appear in quick search results because of newer threads containing same keywords. so basically it becomes hard to locate old threads even though they are active today.
i noticed the problem for the keyword "windows 10". i kind of fixed it by merging some threads and deleting some irrelevant ones. so the primary thread now appear in the quick search results.
forum is running from 2004 and it has collected quite a lot of junk largely because of my own activity over the years. it's not very big of a deal. i can always use google site search to locate older threads. i just like using this tool as the first option.
The problem with sorting by last post date is you would perhaps find the one thread, but most of the other quick searches would provide older threads when most of the time you want the newest threads.
In the example you provided, I would start a new thread called YouTube Updates and close the one started in 2006. That way your members will always get the most recent threads when using the quick search.